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A leader to me is somebody who whatever their background and status, steps up to overcome obstacles and change the world around them and inspires others to do the same.

It's somebody like Anwari Khan, who lives with her children next to one of the biggest rubbish dumps in Asia. A self-taught leader who never learned to read and write, she heads a network of hundreds of women, and runs training courses for the police on responding to domestic violence.

Or Yang Xin, who organised the first expedition from the source to the mouth of the Yangtze river – a journey of almost 4000km. It took 170 days and ten of his fellow explorers died. 25 years on, Yang Xin runs an environmental NGO dedicated to protecting one of the planet's most precious ecosystems – the Qinghai-Tibetan plateau. The rivers that start there support the lives of almost two billion people across south Asia.

Or Taddy Blecher, who quit his job as one of South Africa's highest paid actuaries to set up a low-cost university for people from disadvantaged families. Taddy persuaded a bank to lend him an abandoned building, recruited volunteers from Johannesburg's business community to teach classes, stocked the library with out-of-print textbooks donated by publishers, and equipped the classrooms with computers gifted by software companies. For the first few months, before the computers arrived, he photocopied images of keyboards and taught the students to touch-type on sheets of paper.

Through my work with Leaders' Quest over the last 12 years I've met many unconventional leaders like Anwari, Yang Xin and Taddy.

I decided to write a book about some of them, not because they are inherently more virtuous than high profile leaders who strive to contribute to a happier, healthier world, but because their stories are rarely told and because they too have a vital role to play in creating a brighter future.

They've made me realise that, despite all the challenges we face, we have reason to be optimistic. The potential of human beings to grow, create and overcome adversity is infinite.

But there's also another side to the story. The choices we make can easily be destructive, rather than constructive. I wrote Invisible Giants: changing the world on step at a time, because I wanted to explore some of the tough questions we all face about values, purpose, and the impact we have on the world.

Of course we each need to answer these questions for ourselves. Purpose can't be prescribed. Yet, I've found that certain types of experience consistently catalyse significant mindset shifts for individuals.

I've had the opportunity, through my work at Leaders' Quest, to witness many such moments of revelation, both for myself and for others. I've sat with international bankers and villagers under a tree in Kenya and listened as one of the local farmers asked: "how is it that when the financial crisis came to Europe and America that some of us had our goats and shacks repossessed?"

And I've watched tears roll down the face of a successful Chinese businessman, who, after watching a group of migrant workers perform a harrowing mime about their own life experiences, was coming to terms with the fact that he, albeit indirectly, was part of the system that caused their suffering.

Perhaps William, a young white South African, who, at the age of 17, was sentenced to 24 years in prison for armed robbery, summed it up best. I first met William on a visit to Johannesburg Maximum Security Prison with Khulisa, an organisation that helps young offenders turn their lives around. Thanks to Khulisa, William escaped the cycle of drugs, gangs and crime, and earned early release after just eight years. His positive leadership role within the prison was key to his turnaround and, crucially, he grew to recognise and take responsibility for the impact of his actions.

Shortly after his release, William said to me: "I started to think about what makes us human and how we're all connected. I thought about it like I was a spider in the middle of a web, joined up to all the people in my life as well as lots I'd never even met, and with every one of them I seemed to be a negative force. Facing up to that was hard. I thought about finding myself and then I thought: does anyone ever really do that? I decided it was more a case of working out who I wanted to be and becoming that person."

We all, at least superficially, understand that we live in a complex, interconnected world, but how many of us really have the courage to face up to what that means for us and the decisions we make?